This invention relates to fan drives for motor vehicles and, more particularly, to an improved actuator for a motor vehicular fan drive.
Fan drives for driving the cooling fan of an internal combustion engine are well-known. See, for example, U.S. Pat. No. 3,809,197. Typically, such drives include input and output members which have interdigitated portions or lands and grooves which are spaced closely adjacent each other with a fluid shear medium positioned in the space therebetween. The fluid shear medium functions to transmit the drive from the input member to the output member on which the fan is mounted. The extent to which the fan drive is engaged is dependent on the amount of viscous fluid present in the space between the input and output members and the amount of fluid is controlled by a temperature control valve device. The temperature control valve device includes a temperature sensing element, such for example as a coil spring bimetallic element, disposed at the front of the fan drive in a position to be influenced by the air passing through the radiator of the vehicle. Briefly, when the temperature of the air passing through the radiator is relatively cool, the fan drive is disengaged. When the air temperature passing through the radiator increases to a predetermined value, the temperature sensing element functions to actuate the temperature control valve to pass additional fluid to the space between the clutch members to thereby effect engagement of the fan drive and drive the fan to cool the engine coolant.
Whereas this arrangement is generally satisfactory, it embodies an inherent operational inaccuracy since the temperature of the air impacting the temperature sensing element is typically significantly different than the temperature of the coolant in the radiator. Whereas the temperature at which the temperature sensing element is actuated may be adjusted to accommodate this difference, the adjustment would have to be different for different operating conditions of the same vehicle so that the described arrangement can, at best, be calibrated to be accurate over only a portion of the range of operating conditions encountered by the vehicle. To eliminate this inaccuracy, it has been proposed to sense the temperature of the engine coolant directly and engage the fan drive upon the sensing of a predetermined coolant temperature. However, all such direct coolant temperature sensing systems heretofore proposed have been relatively complex and expensive and/or have necessitated a major redesign of the basic viscous fan drive system.